Most of the semiconductor memory manufacturing companies have developed the MRAM using a ferromagnetic material as one of the next generation memory devices. The MRAM is a memory device to store information by forming multi-layer ferromagnetic thin films. The stored information can be read by sensing current variations according to a magnetization direction of the respective thin film. The MRAM operates at a high speed, reduces power consumption, allows high integration density by the special properties of the magnetic thin film, and performs a nonvolatile memory operation such as a flash memory.
The MRAM embodies memory devices by using the fact that the spin influences electron transmission. First, the MRAM using a giant magneto resistive (GMR) phenomenon utilizes the fact that resistance is larger when spin directions are different in two magnetic layers having a non-magnetic layer therebetween than when spin directions are identical. Second, the MRAM using a spin-polarized magneto-transmission (SPMT) phenomenon utilizes the fact that larger current transmission is generated when spin directions are identical in two magnetic layers having an insulating layer therebetween than when spin directions are different. However, the MRAM research is still in its early stage and mostly concentrated on the formation of multi-layer magnetic thin films, i.e., less on the researches on a unit cell structure and a peripheral sensing circuit.
FIG. 1 is a cross-sectional diagram illustrating a conventional MRAM. Referring to FIG. 1, a source region and a drain region are defined by two N+ regions 13 separately formed on a P-substrate 11. A source contact 17 is formed on the N+ region 13 corresponding to the source region, and a drain contact 19 is formed on the N+ region 13 corresponding to the drain region. The source contact 17 and the drain contact 19 are formed in the same layer as a first interlayer insulating film 21. A gate electrode 15 is separately formed between the source contact 17 and the drain contact 19, and a gate oxide film 14 is formed below the gate electrode 15.
A first contact plug 25 and a second contact plug 27 are formed on the source contact 17 and the drain contact 19, respectively. A ground line 29 and a metal line 33 are formed on the first contact plug 25 and the second contact plug 27, respectively. Accordingly, the ground line 29 and the source contact 17 are electrically connected by the first contact plug 25, and the metal line 33 and the drain contact 19 are electrically connected by the second contact plug 27. A write line 31 is separately formed between the ground line 29 and the metal line 33. The ground line 29, the metal line 33, and the write line 31 are formed in the same layer as a third interlayer insulating film 35.
A third contact plug 39 is formed on the metal line 33 in the same layer as a fourth interlayer insulating film 37. A connection film 41 is formed on the third contact plug 39 to overlap with the write line region. Here, the connection film 41 is formed in the same layer as a fifth interlayer insulating film 43.
An MTJ element 51 is formed on the connection film 41 and in the same layer as a sixth interlayer insulating film 53. The MTJ element 51 has a stacked structure of a pinned ferromagnetic layer 45, a tunnel barrier layer 47, and a free ferromagnetic layer 49. A bit line 55 is formed on the MTJ element 51.
As described above, the conventional MRAM cell is composed of one field effect transistor and one MTJ element. When a voltage is applied to the gate electrode 15 (i.e., word line), the transistor is turned on. As a result, the MRAM cell reads data stored in the MTJ element 51 by sensing the amount of current flowing through the bit line 55. Here, the MTJ element 51 controls the current according to a magnetization direction of the free ferromagnetic layer 49.
In addition, the data can be written by controlling the magnetization direction of the MTJ element 51 in the opposite way. That is, the field effect transistor is turned off, and a current is supplied to the write line 31 and the bit line 55. A magnetic field is generated in response to the current flowing through the write line 31 and the bit line 55 configured to influence the free ferromagnetic layer 49. As a result, the magnetization direction of the MTJ element 51 is controlled. Here, the current is supplied to the bit line 55 and the write line 31 at the same time so that the MTJ cell can be selected from a vertical intersecting portion of two metal lines.
The conventional MRAM must include the write line 31 to write the data on the MTJ element 51. Also, the write line 31 must have at least minimum isolated space from the ground line 29 and the metal line 33, which are formed in the same layer (i.e., the third interlayer insulating film 35). Accordingly, there is a problem in the conventional MRAM as the size of the MRAM cell increases. Further, there is a need for a process to form the write line because of the aforementioned structural problem, which complicates the process for manufacturing the MRAM.